There are also some open-loop systems with special manifolds that allow the cold-water supply to be used as the return line. The pumps shown here are equipped with timers that limit operation to specified periods of demand.

This kit includes a wireless switch for mounting on the counter and a remote receiver for installation in the cabinet. When retrofitted to existing plumbing, the pump is located at the fixture farthest from the hot-water heater.

A pair of tees installed behind the angle stops create an open loop between the hot- and cold-water lines. When the pump is activated, the slug of cool water in the hot line is pushed back to the water heater via the cold-water line. The pump runs only a few seconds, until a sensor detects a rise in the temperature in the hot line.

When possible, the author prefers to install the on-demand circulating pump at the water heater in a closed-loop configuration. The pump can be activated by any number of manual switches or motion detectors located near fixtures on the hot-water line.

Unlike traditional whole-house circulation systems, on-demand pumps can be used with tankless water heaters without voiding the manufacturer's warranty. The author uses a 3/4-inch hot-water return line and increases the pump size to make sure there is enough flow to activate the burner and to account for friction losses within the heater itself.

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When you turn on a hot-water tap, it can take a long time before hot water actually arrives at the faucet. That’s not surprising, considering all the cold water that has to flow out first. For example, a typical bungalow in the San Francisco Bay area — where we work — might have a 60-foot-long 3/4-inch-diameter hot-water supply pipe with an additional 10-foot-long 1/2-inch branch connection to the sink. That much pipe can contain more than 1-3/4 gallons of cold water. Assuming a flow rate of 2.2 gpm, it would take at least 47 seconds before hot water flowed out of the faucet. If the homeowners have a green conscience and have installed a 1.5-gpm reduced-flow aerator on the faucet to save water, they’ll have to wait even longer — 70 seconds — for the hot water to arrive. They’re wasting both water and time.

According to Gary Klein, an expert on water distribution systems, a typical family of four wastes about 10,000 gallons of water per year waiting for hot water. “Average hot-water usage is about 20 gallons per person per day, with a very large variation,” Klein says. “About a third of that, or 7 gallons, is water that runs down the drain while waiting for the hot water to arrive at the tap.” With water and sewer costs averaging about $0.006 per gallon nationwide, 10,000 wasted gallons adds up to about $60 worth of water annually.

Whole-House Circulation

To help solve this problem, my partner, Jim Lunt, and I have installed hundreds of hot-water circulation systems over the past 30 years. When we began, we took the traditional approach: We’d run a dedicated hot-water return leg from the farthest fixture on a home’s plumbing system back to its hot-water heater. These hot-water loops — which are sometimes designed to operate by gravity but more often use a circulating pump — bring heated water directly from the heater to the end of the branch close to each fixture, then return “cooled” hot water back to the heater, where it is reheated. By recirculating water through the loop, the pipes act as an extension of the water heater’s reservoir. Because there is not much cold water to purge from the system, it doesn’t take long for hot water to arrive at the faucet.

The downside of these systems is that the water heater has to fire frequently to maintain that extended reservoir of hot water. To save energy, we’ve used timer-controlled circulating pumps that limit hot-water recirculation to certain hours of the day (see slideshow), or we’ve added aquastats that sense the temperature in the hot-water supply lines so that the pumps won’t operate when there’s already hot water in the pipes. In some cases we’ve combined timer and aquastat controls. These measures help reduce fuel costs, but the homeowner still pays an energy penalty for the convenience of hot-water recirculation.

On-Demand Circulation

These days we primarily install on-demand hot-water systems. We like them because they deliver the hot water rapidly to all the taps on the branch, cutting way down on the amount of cold water wasted down the drain. They offer the same convenience as a whole-house system but without the energy penalty. We install both Metlund (800/638-5863, gothotwater.com) and Taco D’Mand (401/942-8000, taco-hvac.com) systems. Taco makes the circulating pump for both.

On-demand systems feature a high-head pump capable of quickly moving the slug of cold water standing in the hot-water supply pipe toward the farthest faucet on the branch, along with the fully heated water from the water heater behind it. This is in contrast to most whole-house circulation pumps, which are low-head and therefore move the water at much slower flow rates.

How on-demand pumps work. When the on-demand pump comes on, any cool water in the hot supply pipe is pushed into either a dedicated recirculation line toward the water heater (closed-loop configuration) or back into the cold-water supply pipe (open-loop configuration). As soon as the temperature sensor installed in the hot supply detects a 3°F to 5°F rise in the incoming water, the system’s electronic controller shuts the pump off.

Open- vs. Closed-Loop Systems

On-demand pumps are small enough to fit in a bathroom vanity cabinet or below the kitchen sink. We usually place the pump under the faucet that’s farthest from the hot-water heater, especially in a retrofit project. In a simple open-loop — or non-recirculating — installation, the pump module connects to the hot- and cold-water supplies with tee fittings located just before the angle stops. The pump can simply be plugged into the nearest 110-volt outlet. If the electrical circuit is already in place, it usually takes less than two hours to retrofit a house with on-demand hot water.

Hot water return. However, with new construction or during a large renovation, when extensive plumbing work is being done, we prefer to install a dedicated hot-water return leg from the farthest fixture on the branch line back to the water heater, creating a closed loop. This extra plumbing adds cost and isn’t absolutely necessary, but it allows us to place the on-demand pump at the water heater rather than in a bathroom, which eliminates a potentially bothersome source of vibration and noise.

The dedicated leg also eliminates the possibility that small slugs of warmed water will be pushed into the cold-water supply line, which, in our experience, can happen with certain open-loop systems. For example, we’ve found that the temperature-sensitive cartridges in the Grundfos Comfort system, an open-loop whole-house circulation system, often fail, allowing hot water to come out of the cold-water tap. With a hot-water return pipe, this can never happen.